Robinson’s coning hinge

October 27, 2010 by Tim McAdams

In a departure from the traditional semi-rigid rotor system design, Frank Robinson used a coning hinge on each blade when designing the R22 in the 1970s. When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). Although some mistakenly refer to this as a flapping hinge, it is used for blade coning. Previous rotor systems used a fixed coning angle built into each blade. Robinson’s design allows the coning angle to vary according to different conditions such as rotor speed, acceleration, and weight.

Coning via blade bending places a high stress load at the blade’s root. The coning hinge relieves this stress, reducing the amount of reinforcing required and making for a lighter, easier to manufacture rotor blade. Additionally, the reduced bending of the rotor blade at its root allows the pitch-change axis to be better aligned with the blade’s centerline. This reduces the forces across the hub, pitch change bearings, and the rotor blades and, as such, decreases shake and feedback in the cyclic control.

Known as a tri-hinge rotor hub (the third hinge is called a teetering hinge and is typically the only hinge in a semi-rigid rotor system) Robinson was granted a patent for it in 1978. The same hub design (although larger in size) is used on the R44 and the new turbine-powered R66.

  • Ehud Gavron

    This is the reason the blades say “Never pull down” and the tiedowns say “Never pull down.” People often think the wings can’t take being pulled, but any blade that can lift a 1370 or 2500lb helicopter can handle a 200lb man pulling 😉

    The real reason is when a blade is pulled down, the fulcrum of the coning hinge means one has to use 75x the power to get the teetering effect. (That number from RHC Safety Course). On the other hand, pushing up just uses the teetering hinge, and only 1x the power :)


  • pdxpilot

    Good stuff – I started my private training in a R22 and this was never explained to me. Thanks for keeping this going – will you be at Summit next month?

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  • Jean-Pierre Harrison

    AOPA has established an uneviable record of not knowing what it is talking about when pontificating about rotary-wing aircraft. This blog post is just another example.

    R-22 ROTOR SYSTEM ~ Posting by Frank Robinson on www, ~ Nov. 29, 2000.

    I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle. First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick. Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft. The R22 rotor system was designed with 18 degrees of delta-three to eliminate two minor undesirable characteristics of rotor systems having 90-degree pitch links. In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as “wee-wa.” This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive. If no lateral cyclic was applied, this would result in a rotor disc tilt to the right while the rotor plane was tilting forward. Pilots subconsciously learn to compensate for this by applying some lateral cyclic as the cyclic is being moved forward. The amount of delta-three required to eliminate “wee-wa” in the R22 rotor system was calculated to be 19 degrees. The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade’s angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. The amount of delta-three required to compensate for this effect in the R22 rotor system was calculated to be 17 degrees. A delta three angle of 18 degrees was selected as the best compromise angle to reduce or eliminate the two undesirable characteristics described above, which would have been present in the R22 had a 90-degree pitch link design been used. Subsequent instrumented flight test data confirmed the choice of the 18-degree delta-three angle. Hopefully, this will help clarify a few of the misconceptions concerning the design of the R22.

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